Switching Class D audio amplifiers have found increasing use in the industry in recent years, due to the improvements in output stage switching devices and equally modulation and feedback control methods. The classical switching power amplifier system consists of the pulse modulator, converting an analog or digital source into a pulse modulated signal, following amplified by a switching power stage. A passive demodulation filter reproduces the power modulated power signal.
Most switching class D amplifiers are based on variants of Pulse Width Modulation. The challenges in switching amplifier design relate to:
PWM is in effect a multiplication/mixing between the input and power supply variable. This is equivalent to zero power supply rejection.
The switching power stage cause distortion from numerous contributions, since power MosFETs have parasitics and need to be driven by differentiated turn-off/turn-on delays.
The output filter is non-linear and contributes with significant addition of frequency dependent output impedance, which counters the desire for ideal voltage control of the speaker load.
EMI. The power stage, passive filter and the connecting cables (although filtered) source EMI. Perfect demodulation is not possible, leaving residuals on connected cables.
Achieving Robust Stability & Excellent audio performance is complicated, given the real world and test bench parameter space for load perturbations, input stimuli and power supply range.
In general, effective feedback control systems have proven vital to reach performance and robustness on par with legacy class AB amplifiers. Also, feedback control can be utilized to drive efficiency up and complexity down, as efficiency and complexity are determined by the power stage and demodulation filter.
Pulse modulation may be implemented with classical carrier based PWM or PDM modulation or by utilizing self-oscillation methods. The overall shortcomings of carrier based PWM switching power amplifiers have been extensively covered in the Ph.D thesis “Audio power amplifier techniques with energy efficient power conversion” by the inventor. In order to overcome classical PWM switching power amplifier drawbacks, a controlled oscillating modulator (COM), in effect a feedback oscillation modulator, was introduced in the international patent application WO98/19391. In combination with the an enhanced cascade feedback method, a range of the Class D shortcomings outlined above were solved.
Other oscillating modulator methods have been disclosed in prior art as WO2004/47286 by the applicant, WO 03/090343 and U.S. Pat. No. 6,489,841. These methods are characterized by self-oscillation being determined by feedback after the output filter, i.e. having the output filter as an integral, determining part on self-oscillation conditions. Such architectures will in the following be reference to as global loop oscillation modulators. The global loop oscillation modulator based switching amplifier systems disclosed in prior art have a particular advantage in terms of maximized loop gain-bandwidth enclosing the output filter, such that filter distortion and output impedance is minimized.
One serious problem however, by enclosing the filter inside the loop determining oscillation conditions, is that oscillation conditions become filter Q dependent. This generally introduces a load conditioned stability in the system, unless the filter is damped passively or the system compensated by other means. In particular load situations, open load or capacitive loads, corresponding to a full 180 deg phase lag at the filter natural frequency in case the filter is 2nd order, will generally introduce a 2nd oscillation state in the proximity of the filter natural frequency. Oscillation at the filter natural frequency, in a high filter Q load situation, is absolutely unacceptable, and will generally lead to system damage. Subsequently, passive filter damping with RC Zoebel networks to reduce filter Q in open load situations is a solution, however power resistors add complexity to the system and degrade efficiency. Excessive loop compensation by e.g. feedback path differentiation is an alternative attempted in prior art, however this reduces the effective loop transfer function gain. In effect, a 0th order compensation system is needed around the filter in order to prevent the undesired 2nd state oscillation around the filter natural frequency.
A second disadvantage of global loop oscillation modulators is that the feedback differentiation needed in order to improve stability, generally disturbs control system implementation. Feedback differentiators pick up noise and feed it to the control system typically consisting of linear opamps. A further disadvantage is the feedback differentiation, is the effect of introducing a pole or several poles in the system transfer function, thus limiting bandwidth. The feedback lead or differentiation puts put restrictions on amplifier design. As such it is impossible to design for both high performance, robustness to load perturbations and high efficiency over the complete audio band.
A third disadvantage of this prior art global loop oscillation modulator architecture is that power stage and power supply related errors are generally not corrected locally. The significant error sources introduced by the switching power stage and power supply need to pass the 2nd or higher order output filter phase shift and delay, before generating the error signal for overall compensation. As such, the improved filter compensation generally compromises power stage and power supply related error compensation.